Boronate-urea catalysts

The c-BN phase was first obtained in 1957 [525] by exposing hexagonal boron nitride phase (h-BN) to high pressures and low temperatures. A pressure of more than 11 GPa is necessary to induce the hexagonal to cubic transformation, and these experimental conditions prevent any practical application for industrial purposes. Subsequently, it has been found that the transition pressure can be reduced to approximately 5 GPa at very high temperature (1300-1800°C) by using catalysts such as alkali metals, alkali metal nitrides, and Fe-Al or Ag-Cd alloys [526-528]. In addition, water, urea, and boric acid have been successfully used for synthesis of cubic boron nitride from hexagonal phase at 5-6 GPa and temperature above 800-1000°C [529]. It has been... 

The reaction is acid or base catalyzed. Many catalysts have been tried, including potassium acetate and sodium acetate (27), dimeth-ylformamide (DMF) (28-30), urea ammonium sulfate (29), magnesium perchlorate (31-33), trifluoroacetic acid (32), boron trifluoride (30), sodium acetate (31), potassium hydrogen phosphate (34), and y-rays (35). The best acetylation condition, however, is uncatalyzed acetic anhydride in xylene at 100-130 C (36). 

Reports by the groups of Chan, Evans, and Lam in 1998 revealed an alternative method to conduct copper-mediated couplings that form C(aryl)-0 and C(aryl)-N bonds. In this process, arylboronic acids react with compounds containing N-H or 0-H bonds in the presence of a Cu(II) reagent or catalyst. TTiese reactions were initially conducted with stoichiometric amounts of copper reagents. " Amines, anilines, amides, ureas, carbamates, and sulfonamides underwent N-arylation in moderate to excellent yields by this process (Equation 19.124). The commercial availability of boronic acids and the ability to conduct these arylations in air under mild conditions has caused this method to be adopted quickly for synthetic applications on a small scale. 

The O.S.W. process is shown in Fig. 28.22. Urea, along with recycle ammonia, is fed into a fluidized-bed reactor containing inert solids, where the temperature is raised to 350°C within a few seconds. The resulting mixture of isocyanic acid and ammonia then is fed into a reactor containing a solid catalyst, such as silica gel, alumina, or boron phosphate. Products from the isocyanic acid reactor are quenched with water, the resulting slurry is centrifuged, and the mother liquor is sent to recovery or a urea plant. 

A very mild copper-catalyzed vinylation of amines, which involved the use of advantageous vinyl boronic acids at room temperature, was developed by Lam (Scheme 21) [89]. They proved that a combination of Cu(OAc)2 and pyridine A-oxide was an efficient catalyst system for the coupling of tra/w-1-hexenyl boronic acid with certain amines. In this respect, not only A-heterocycles such as benzimidazole and indazole but also numerous ureas and lactams underwent the cross-coupling under similar conditions. 

A more reactive catalyst pair is provided by placing the Lewis acid and Lewis basic phosphorus and boron moieties in vicinal carbon atoms, such as in catalyst 97 (Scheme 6.18) [37]. This design is very similar to that of (thio)urea-tertiary amine catalysts discussed in the preceding section. Using this catalyst 97, enamines can also be reduced by the FLP catalyst. These reactions likely proceed via the intermediacy of the corresponding iminium ions. 

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